REGIONAL DIFFERENCES IN RABBIT ATRIAL REPOLARIZATION - IMPORTANCE OF TRANSIENT OUTWARD CURRENT

Regional differences in rabbit atrial repolarization were investigated using a conventional microelectrode technique. A more rapid phase 1 r epolarization (lower phase 1 amplitude) was seen in the left atrial (L A) roof area compared with the right atrial (RA) roof area: 54 +/- 10 vs. 82 +/- 6 mV at 1,000 ms (P < 0.001). In addition, action potential duration at 40 mV above the resting potential (APD(40)) was shorter i n LA and was associated with a slower phase 3 repolarization rate. Fur thermore, the recovery time constant of phase 1 amplitude at 500 ms wa s 0.9 +/- 0.2 s in LA and 3.5 +/- 1.5 s in RA (P < 0.001). Pacing cycl e lengths (2,000, 1,500, 1,000, 800, and 500 ms) modulated phase 1 amp litude, APD(40), and phase 3 rate in both regions. 4-Aminopyridine (4- AP; 1 mM), a selective transient outward current (I-to) blocker, aboli shed cycle length dependence of the above action potential parameters and diminished the differences in electrophysiological properties betw een the two regions. 4-AP also flattened the restitution curve of phas e 1 amplitude in both regions. In conclusion, the findings suggest tha t the different kinetics of I-to, play an important role in regional d ifferences of atrial repolarization.